1. Field of the Invention
This invention relates to electron-beam exposure systems that form fine patterns of integrated circuits on substrates such as semiconductor wafers, which are exposed to electron beams. Particularly, this invention relates to electron-beam exposure systems which are suitable for cell projection to obtain high throughputs.
This application is based on Patent Application No. Hei 10-331790 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
As compared with the conventional reduction projection techniques using ultraviolet radiation, electron-beam exposure techniques are advantageous in that very fine patterns can be formed on semiconductor wafers with high resolution. Recently, it is demanded that semiconductor devices such as memories should be manufactured with high throughputs. To cope with such a demand, the electron-beam exposure systems employ the cell projection using masks specifically designed for electron beams.
According to the cell projection, a part of a LSI pattern is repeatedly imaged on a substrate in accordance with an opening shape of an aperture. This system is advantageous particularly in manufacture of memory devices such as DRAMs (i.e., dynamic random-access memories), which contain plenty of repeated patterns, because a number of times to perform exposure can be remarkably reduced. So, as compared with the conventional variably shaped electron-beam exposure which exposes patterns in such a way that a picture is drawn with a single rectangular, it is possible to remarkably reduce a writing time.
An example of the conventional electron-beam exposure system will be described with reference to FIG. 9. Herein, an electron source 1 radiates an electron beam, a square shape of which is formed by a first aperture 2. The electron beam is converged by first and second electromagnetic lenses 3, 4. Then, the electron beam is irradiated onto an electron-beam mask 5 having an opening pattern 5a. The electron beam transmitted through the opening pattern 5a of the mask 5 is subjected to reduction and convergence by a third electromagnetic lens 6. Then, it is transmitted through an object aperture 7 and is subjected to convergence by an objective electromagnetic lens 8. Thus, the electron beam is finally irradiated on a surface of a semiconductor wafer W.
Incidentally, the electron source 1 of the conventional type is a square surface electron source as shown in FIGS. 10A and 10B, in which a radiation surface 1a of the electron beam is formed like a flat plane. Or, the conventional electron source employs another type of the radiation surface 1a whose a center portion is projected. In FIG. 9, opening portions of prescribed sizes are formed at center portions of the aforementioned apertures 2 and 7 to allow transmission of the electron beam. That is, the first aperture 2 has an opening portion 2a, while the object aperture 7 has an opening portion 7a.
The aforementioned electron-beam exposure technique suffers from problems, as follows:
In the cell projection, a pattern of a large area is imaged on the substrate at once. This increases a beam current being transmitted through the lenses and apertures. Increase of the beam current causes a so-called Coulomb effect to become large, so that an accuracy for formation of a beam shape is deteriorated due to repulsion being effected between electrons in response to an amount of the beam current. There is a disadvantage in which the beam is broadened in size so that resolution is reduced when the Coulomb effect becomes large. For this reason, it is impossible to increase the beam current so much. Therefore, it is difficult to improve the throughput. Conventionally, the electron beams radiated from the electron source have Gaussian distribution in distribution of electron-beam intensities which are distributed in connection with beam incident semi-angles of the electron beams being irradiated on the substrate. So, electrons are concentrated at a center portion of the section of the electron beam. Therefore, the Coulomb effect caused by increasing the beam current influences the conventional electron-beam exposure technique more badly as the beam incident semi-angle becomes smaller.